The door speakers seem really delicate or maybe it's because the driver door gets open/closed a lot which damages the speaker... I had a buzz on high frequencies from the door speaker and had it replaced.

Make sure it is the speaker itself and not the wiring that has come loose from behind the speaker. You have to remove the door panel, but the wires should all be taped in place from the factory. This can be a source of rattles or buzzing if they are not taped down.

try this, when it starts vibrating, put your thumb on the center of the grille and apply pressure
mine used to do that, it was the metal part of the grill that was vibrating against the plastic circular frame of the grille
if you look closely, there is some type of glue on the inner circle of the cover that is meant to hold the metal grille in place
i don't think that does the job properly
on the HIFI setup, there is a thin fabric cover between the metal grills and the plastic frame
on the enhanced, there is no such fabric.

Well, I posted this thread to see if it was indeed an issue. The first dealership I took it to wouldn't replace it under warranty. I even demonstrated the buzz and he just said I had the speakers on too loud (?). Luckily another dealership took care of it no questions asked.